Fix many warnings/errors when building Observatory

runtime/observatory/tests/observatory_ui/mocks/objects/heap_snapshot.dart

Index: runtime/observatory/tests/observatory_ui/mocks/objects/heap_snapshot.dart
diff --git a/runtime/observatory/tests/observatory_ui/mocks/objects/heap_snapshot.dart b/runtime/observatory/tests/observatory_ui/mocks/objects/heap_snapshot.dart
index d0b268d6a5300f567e47c108874cdd220258715c..38cde90adebdcb408c53da59eb6bb85554927382 100644
--- a/runtime/observatory/tests/observatory_ui/mocks/objects/heap_snapshot.dart
+++ b/runtime/observatory/tests/observatory_ui/mocks/objects/heap_snapshot.dart
@@ -10,6 +10,7 @@ class HeapSnapshotMock implements M.HeapSnapshot {
   final int references;
   final int size;
   final M.HeapSnapshotDominatorNode dominatorTree;
+  final M.HeapSnapshotMergedDominatorNode mergedDominatorTree = null;
   final Iterable<M.HeapSnapshotClassReferences> classReferences;
 
   const HeapSnapshotMock(
@@ -24,6 +25,7 @@ class HeapSnapshotMock implements M.HeapSnapshot {
 class HeapSnapshotDominatorNodeMock implements M.HeapSnapshotDominatorNode {
   final int shallowSize;
   final int retainedSize;
+  final bool isStack = false;
   final Future<M.ObjectRef> object;
   final Iterable<M.HeapSnapshotDominatorNode> children;